Direct sequence modulation is a type of radio frequency modulation wherein a data stream is spread by a higher rate bit stream prior to modulation. A prior art direct sequence modulation transmitter includes a radiation device (antenna), a modulator or demodulator, and a data stream generator. In addition, a prior art transmitter generally includes a modulation key generator that is used to control the modulation characteristics of a radio frequency (RF) signal.
Direct sequence modulation is widely used in order to enhance security and to minimize interference and noise. A spread spectrum transmitter can transmit in such a way as to minimize transmission power and interference and maximize data security. As a result, a direct sequence modulation communication system often includes security features such as encryption, frequency hopping, and time hopping. Furthermore, the direct sequence modulation communication system often mixes an input data stream with a pseudorandom noise (PN) stream in order to spread out the signal bandwidth and decrease the signal to noise ratio. This is commonly done in order to make the signal indistinguishable from background noise in order to hide it from potential eavesdroppers. As a result, a transmission may advantageously have reduced interference and noise vulnerability and increased security as the data stream may be spread over a larger instantaneous bandwidth and/or over a range of frequencies. This makes the signal hard to intercept or jam.
A prior art communication device operates by generating a modulation key that controls one or more RF modulation characteristics. The prior art modulation key is supplied to the transmitter, along with the data stream input. The transmitter modulates the data stream onto a carrier wave according to the modulation characteristics of the modulation key. The prior art receiver also includes a modulation key generator, and demodulates a received RF signal according to the same modulation characteristics.
The drawback of the direct sequence modulation of the prior art is the requirement of a modulation key generator in the transmitter and receiver. The modulation key generator consumes extra electrical power and requires extra circuitry, extra complexity, extra weight, extra size, and extra cost. These are typically concerns in direct sequence modulation transmitters, and especially in portable communication devices and satellite-based communication devices.
What is needed, therefore, are improvements in such communication devices.